This application claims benefit of Provisional Application Ser. No. 60/079,871 filed Mar. 30, 1998.
The present invention relates to a heat-sensitive imaging element for making lithographic printing plates. More specifically the invention relates to a heat-sensitive imaging element which requires no processing.
Lithographic printing is the process of printing from specifically prepared surfaces, some areas of which are capable of accepting ink, whereas other areas will not accept ink.
In the art of photolithography, a photographic material is made imagewise receptive to oily or greasy ink in the photo-exposed (negative working) or in the non-exposed areas (positive working) on a ink-repelling background.
In the production of common lithographic plates, also called surface litho plates or planographic printing plates, a support that has affinity to water or obtains such affinity by chemical treatment is coated with a thin layer of a photosensitive composition. Coatings for that purpose include light-sensitive polymer layers containing diazo compounds, dichromate-sensitized hydrophilic colloids and a large variety of synthetic photopolymers. Particularly diazo-sensitized systems are widely used.
Upon imagewise exposure of such light-sensitive layer the exposed image areas become insoluble and the unexposed areas remain soluble. The plate is then developed with a suitable liquid to remove the diazonium salt or diazo resin in the unexposed areas.
On the other hand, methods are known for making printing plates involving the use of imaging elements that are heat-sensitive rather than photosensitive. A particular disadvantage of photosensitive imaging elements such as described above for making a printing plate is that they have to be shielded from the light. Furthermore they have a problem of stability of sensitivity in view of the storage time and they show a lower resolution. The trend towards heat-sensitive printing plate precursors is clearly seen on the market.
EP-A-444 786, JP-63-208036, and JP-63-274592 disclose photopolymer resists that are sensitized to the near IR. So far, none has proved commercially viable and all require wet development to wash off the unexposed regions. EP-A-514 145 describes a laser addressed plate in which heat generated by the laser exposure causes particles in the plate coating to melt and coalescence and hence change their solubility characteristics. Once again, wet development is required.
EP-A-652 483 discloses a lithographic printing plate requiring no dissolution processing which comprises a substrate bearing a heat-sensitive coating, which coating becomes relatively more hydrophilic under the action of hat. Said system yields a positive working printing plate. EP-A-609 941 describes a heat-mode recording material comprising on a substrate a metallic layer and a thin hydrophobic layer which becomes hydrophilic upon exposure. However the lithographic performance of the obtained printing plate is poor.
It is an object of the present invention to provide a heat-sensitive imaging element for preparing lithographic printing plates requiring no dissolution processing and having a high lithographic performance (ink acceptance, scratch resistance, durability)
According to the present invention there is provided a heat-sensitive imaging element for making lithographic printing plates comprising on a lithographic base, having a hydrophilic surface, a metallic layer or metal oxide layer and on top thereof an oleophobic  oleophilic polymer layer having a thickness of less than 5 xcexcm characterised in that said polymer layer comprises a polymer containing phenolic groups.
It has been found that according to the present invention, using an imaging element as described above, lithographic printing plates requiring no processing and having an excellent lithographic performance can be obtained.
Metallic layers or metal oxide layers suitable for use in accordance with the invention comprise metals or metal oxides converting the actinic radiation to heat so that the oleophobicity  oleophilicity of the oleophobic  oleophilic top-layer is destroyed. The thickness of the metallic layer or metal oxide layer is preferably from 0.01 xcexcm to 2 xcexcm, and most preferably from 0.05 xcexcm to 1.5 xcexcm. Specific examples of metal layers or metal oxide layers are aluminum, titanium oxide, bismuth and silver of which the three latter are preferred.
A silver layer for use in this invention as the metallic layer can be made according to the principles of the silver complex diffusion transfer reversal process, hereinafter called DTR-process, having been described, e.g. in U.S. Pat. No. 2,352,014 and in the book xe2x80x9cPhotographic Silver Halide Diffusion Processesxe2x80x9d by Andrxc3xa9 Rott and Edith Weydexe2x80x94The Focal Pressxe2x80x94London and New York, (1972).
In the DTR-process non-developed silver halide of an information-wise exposed photographic silver halide emulsion layer material is transformed with a so-called silver halide solvent into soluble silver complex compounds which are allowed to diffuse into an image-receiving element and are reduced therein with a developing agent, generally in the presence of physical development nuclei, to form a silver image having reversed image density values (xe2x80x98DTR-imagexe2x80x99) with respect to the black silver image obtained in the exposed areas of the photographic material.
In another method for providing a metal layer on the lithographic base having a hydrophilic surface a silver halide emulsion disposed on a lithographic base having a hydrophilic surface is strongly exposed to actinic radiation and then developed, or otherwise processed to maximum blackness. The black opaque emulsion is converted to a reflective recording material by heating at least to 270xc2x0 C. in an oxygen containing environment until the emulsion coating assumes a shiny reflective appearance. Such method is disclosed in U.S. Pat. No. 4,314,260.
According to an alternative method for providing a metal layer on the lithographic base the metal is provided using vapour or vacuum deposition.
According to another embodiment of the invention the metallic layer can be a bismuth layer that can be provided by vacuum deposition.
A drawback of the method of preparation of a thin bismuth recording layer by vacuum deposition is the fact that this is a complicated, cumbersome and expensive process.
Therefore, in EP-A-97201282 the vacuum deposition is replaced by coating from an aqueous medium. According to this disclosure a thin metal layer is formed by the following steps:
(1) preparing an aqueous medium containing ions of a metal,
(2) reducing said metal ions by a reducing agent thus forming metal particles,
(3) coating said aqueous medium containing said metal particles on a transparent support.
As a metal oxide layer preferably a titanium oxide layer is used. This layer can be applied to the substrate by vacuum deposition, electron-beam evaporation or sputtering.
The oleophobic  oleophilic layer provided on top of the metallic layer or metal oxide layer comprises a polymer containing phenolic groups. Preferred polymers containing phenolic groups are phenolic resins (e.g. novolac) or hydroxyphenyl substituted polymers (e.g. polyhydroxystyrenes). The oleophobic  oleophilic layer has a thickness of less than 5 xcexcm. As a consequence a highly sensitive heat-sensitive imaging element is obtained. The use of a polymer containing phenolic groups furthermore improves the lithographic performance (ink acceptance, scratch resistance, durability) of the lithographic printing plates obtained according to the present invention.
According to one embodiment of the present invention, the lithographic base having a hydrophilic surface can be an anodised aluminum. A particularly preferred lithographic base having a hydrophilic surface is an electrochemically grained and anodised aluminum support. Most preferably said aluminum support is grained in nitric acid, yielding imaging elements with a higher sensitivity. According to the present invention, an anodised aluminum support may be treated to improve the hydrophilic properties of its surface. For example, the aluminum support may be silicated by treating its surface with a sodium silicate solution at elevated temperature, e.g. 95xc2x0 C. Alternatively, a phosphate treatment may be applied which involves treating the aluminum oxide surface with a phosphate solution that may further contain an inorganic fluoride. Further, the aluminum oxide surface may be rinsed with a citric acid or citrate solution. This treatment may be carried out at room temperature or can be carried out at a slightly elevated temperature of about 30 to 50xc2x0 C. A further increasing treatment involves rinsing the aluminum oxide surface with a bicarbonate solution. Still further, the aluminum oxide surface may be treated with polyvinylphosphonic acid, polyvinylmethylphosphonic acid, phosphoric acid esters of polyvinyl alcohol, polyvinylsulphonic acid, polyvinylbenzenesulphonic acid, sulphuric acid esters of polyvinyl alcohol, and acetals of polyvinyl alcohols formed by reaction with a sulphonated aliphatic aldehyde. It is further evident that one or more of these post treatments may be carried out alone or in combination.
According to another embodiment in connection with the present invention, the lithographic base can comprise a flexible support, such as e.g. paper or plastic film, provided with a hardened hydrophilic layer. A particularly suitable hardened rough hydrophilic layer may be obtained from a hydrophilic binder hardened with a hardening agent such as formaldehyde, glyoxal, polyisocyanate or preferably a hydrolysed tetra-alkylorthosilicate.
As hydrophilic binder there may be used hydrophilic (co)polymers such as for example, homopolymers and copolymers of vinyl alcohol, acrylamide, methylol acrylamide, methylol methacrylamide, acrylic acid, methacrylic acid, hydroxyethyl acrylate, hydroxyethyl methacrylate or maleic anhydride/vinylmethylether copolymers.
A hardened hydrophilic layer on a flexible support used in accordance with the present embodiment preferably also contains substances that increase the mechanical strength and the porosity of the layer e.g. colloidal silica. In addition inert particles of larger size than the colloidal silica can be added e.g. silica prepared according to Stober as described in J. Colloid and Interface Sci., Vol. 26, 1968, pages 62 to 69 or alumina particles or particles having an average diameter of at least 100 nm which are particles of titanium dioxide or other heavy metal oxides. Incorporation of these particles gives the surface of the hardened hydrophilic layer a uniform rough texture consisting of microscopic hills and valleys.
The thickness of the hardened hydrophilic layer may vary in the range of 0.2 to 25 xcexcm and is preferably 1 to 10 xcexcm.
Particular examples of suitable hardened hydrophilic layers for use in accordance with the present invention are disclosed in EP-A601 240, GB-P-1 419 512, FR-P-230 354, U.S. Pat. No. 3,971,660, U.S. Pat. No. 4,284,705 and EP-A 514 490.
As support on which the hydrophilic layer is provided it is particularly preferred to use a plastic film e.g. substrated polyethylene terephthalate film, cellulose acetate film, polystyrene film, polycarbonate film etc . . . . The plastic film support may be opaque or transparent.
It is particularly preferred to use a polyester film support to which an adhesion improving layer has been provided. Particularly suitable adhesion improving layers for use in accordance with the present invention comprise a hydrophilic binder and colloidal silica as disclosed in EP-A-619 524, EP-A-620 502 and EP-A-619 525. Preferably, the amount of silica in the adhesion improving layer is between 200 mg per m2 and 750 mg per m2. Further, the ratio of silica to hydrophilic binder is preferably more than 1 and the surface area of the colloidal silica is preferably at least 300 m2 per gram, more preferably a surface area of 500 m2 per gram.
In accordance to the method of the present invention for obtaining a lithographic printing plate the heat-sensitive imaging element is image-wise scanning exposed using a laser, preferably a laser that operates in the infrared or near-infrared, i.e. wavelength range of 700-1500 nm. Most preferred are laser diodes emitting in the near-infrared.
After the exposure the imaging element can be used without an additional wet treatment as a lithographic printing plate.
The printing plate obtained according to the present invention can also be used in the printing process as a seamless sleeve printing plate. This cylindrical printing plate wich has as diameter the diameter of the print cylinder is slided on the print cylinder instead of applying in a classical way a classically formed printing plate. More details on sleeves are given in xe2x80x98Grafisch Nieuwsxe2x80x99 ed. Keesing, 15, 1995, page 4 to 6.
The following examples illustrate the present invention without limiting it thereto. All parts and percentages are by weight unless otherwise specified.